The present invention relates to a fault detector system for starting a plasma arc working apparatus.
Plasma arc working apparatus of the type to which the present invention pertains are widely used in applications for cutting metallic workpieces. Typical plasma arc working apparatus are disclosed in U.S. Pat. No. 2,806,124 to R. M. Gage and U.S. Pat. No. 3,366,772 to R. J. Wickham et al. Such apparatus usually comprise a torch having a non-consumable electrode and a nozzle. An electric arc is established between the electrode and the workpiece from a suitable DC power supply. Gas passes through the torch around the electrode and the arc and out through a constricting passage in the nozzle. The gas is heated by the energy of the arc to create a plasma stream which combines with the arc to cut the workpiece. Since the plasma arc reaches extremely high temperatures, a coolant such as water is passed through the nozzle around the arc constricting passage. Coolant may also be passed around the plasma arc to further constrict the arc and prevent damage to the nozzle by high temperatures.
Most plasma arc torches are conventionally started by a small pilot arc in close proximity to the torch electrode. The pilot arc is maintained directly in the path of the gas as it passes around the electrode. With an arc voltage established between the electrode and workpiece from the DC power supply, the pilot arc ionizes a portion of the gas which facilitates transfer of the pilot arc to the main arc between the electrode and workpiece. The pilot arc is ignited and maintained from a separate pilot arc current source and a high frequency AC generator. Once the main arc has been established, the pilot arc is extinguished.
Certain malfunctions may occur from time to time during the starting operation of a plasma arc working apparatus. A typical malfunction may be, for example, a failure to establish a main arc voltage from the DC power supply with the result that no main arc is struck. Another typical malfunction may be a failure to ignite the pilot arc. Still another malfunction may be a failure of the pilot arc once ignited to transfer the main arc across the torch electrode and workpiece. Other malfunctions may also occur such as a failure in the flow of gas and coolant through the torch.
When a fault or malfunction occurs during the starting operation, it is necessary for the operator to completely shut down the plasma arc working apparatus. If the proper procedures are not followed, damage to the apparatus may occur and the operator may be exposed to the hazards of dangerously high voltages.